Mostly money, I believe and perhaps a lingering sense of it not being as "prestigious" as the other majors
Makes sense, given the huge fuss over professionals and amateurs that had gone on for most of tennis history
In the Open Era, Kodes (3 Slams - Wimbledon, 2 Frenches), Smith (2 Slams - Wimbledon, US), Nastase (2 Slams - French, US) never competed at Aus.
Connors did twice, Borg once. McEnroe twice during his prime (played more regularly when he was washed up and managed to get himself disqualified one year)
Lendl skipped one year citing a grass allergy, which strangely never kept him from SW19
Edberg, who won the last two editions played on grass (85 & 87), considered his Wimbledon title in 1988 to be his first "real" Major
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Didn't have a 128 man field until 1988. Prior to that year, very rarely were all matches best of 5 (all the Slams, bar Wimbledon have experimented with Bo3 in in early rounds - none to remotely like the extent as Aus)
There were byes in early rounds and the choice of who got those byes seems to have been pretty random
One year, they had Bo3 matches for a couple of round, than Bo5 for one round, than Bo3 again for a couple of rounds, than Bo5 for the last few rounds
A joke for a lot of reasons, but I imagine at the root of it all was -